Exemplary embodiments of the present invention relate generally to a method for operating a metal detection system and to a metal detection system operating according to this method.
In the industry where machinery is involved in the production of goods, there is always a possibility that a piece of metal, such as a screw or a bolt, does break away from the machinery, and finally ends up in the processed product. Therefore metal detection systems are used at various stages of a production process to detect products that are contaminated by metal. Metal detection systems are also often used for inspecting finished product, in order to ensure consumers' safety and quality standards.
Most modern metal detection systems utilize a search head comprising a “balanced coil system” that comprises three coils, one transmitter coil and two receiver coils that are aligned in parallel. During the inspection process, the product, typically transported on a conveyor belt, is passed through the coils of the “balanced coil system”. In the transmitter coil, which is placed between the receiver coils, flows an electrical current that generates an alternating magnetic field that induces an electrical signal in the two receiver coils. The receiver coils are positioned symmetrically to the transmitter coil, so that identical signals are induced in both receiver coils when no product is present in the “balanced coil system.” In addition, the receiver coils are coupled together in such a manner that the signals induced therein are subtracted from each other. In that way, when no product is present in the balanced coil system, there is a zero signal at the output of the receiver coils. However, a piece of magnetically and/or electrically conductive material, that passes through the balanced coil system, will disturb the magnetic field and will cause modifications of the electrical signal(s) induced in the receiver coils. These perturbations occur first in the first receiver coil and then in the second receiver coil, when the product approaches it. As a result, an electrical signal with a specific phase and amplitude will appear at the output of the receiver coils, when the product passes through the “balanced coil system”. Each magnetic and/or conductive material passing through the metal detection system creates a different signal, according to its conductivity, its magnetic permeability, its shape, its size and its orientation relative to the receiver coils.
To detect the presence of metal in the product, the signals induced in the receiver coils are processed in the receiver stage that typically comprises an input amplifier. In a further stage, the processed signals are analyzed in phase and amplitude in order to detect metal contamination. Finally, the results are displayed on a user interface and/or signaled to the control system.
Various types of metals that are used in machinery, including ferrous (iron), non-ferrous (e.g. copper, aluminum, brass) and various types of stainless steel, may appear as a contaminant in a processed product. If such a metal has a high magnetic permeability, like ferrite, it will primarily be reactive, that means that its signal phase will be close to zero, while a metal with a low magnetic permeability will primarily be resistive and have a signal phase close to 90 degrees relative to the phase of the transmitter signal. Ferrous metals are easily detectable because of their small phase difference with the transmitter signal. Contaminant materials with a high conductivity can easily be detected if the inspected product is dry. On the other hand, non-ferrous metals and particularly stainless steel are difficult to detect in wet products since their phase is similar to the product phase.
However, not every metal passing through a metal detection system is a contaminant, since it could be part of the product packaging. During inspection, the product is often in its final state and already packed. It can be wrapped in a metalized film, typically a plastic film coated with aluminum. This electrically conductive metal of the product packaging creates a signal in the metal detection system that must not be confused with a signal caused by a metal contaminant. Hence in order to detect a contaminated product, it is required that the metal detection system is capable of distinguishing between signals originating from packaging material and signals originating from metal contaminants.
Further, not all disturbances of the magnetic field of the receiver coils are caused by products and metal contaminants traveling through the “balanced coil system.” Vibrations of a conductive material near the balanced coil system may also cause signal changes in the receiver coils that need to be distinguished from signals caused by contaminated products. Signals caused by vibrations are primarily in phase with the transmitter signal.
Food products like cheese, fresh meat, warm bread, jam, and pickles are generally electrically conductive if they contain water, salt, or acid. Therefore, such products traveling through the balanced coil system also disturb the magnetic fields, thus causing a signal at the output of the receiver coils. In order to avoid a false rejection of a product, the product signal needs to be compensated or eliminated.
Hence, for a reliable product inspection, signals caused by vibrations, the product and the packaging have to be eliminated so that only signals are considered that are caused by metal contaminants. However, it has been found that the phase and the magnitude of the signals caused by the product and the metal contaminants depend on the applied transmitter frequency.
In known systems, the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
U.S. Pat. No. 5,994,897A, for example, discloses an apparatus that is capable of switching between at least two different transmitter frequencies such that any metal particle in a product will be subject to scanning at different frequencies. The frequency of operation is rapidly changed so that any metal particle passing through on a conveyor belt will be scanned at two or more different frequencies. In the event that for a first transmitter frequency the signal component caused by a metal particle is close to the phase of the signal component of the product and thus is masked, then it is assumed that for a second frequency, the phase of the signal component caused by the metal particle will differ from the phase of the signal component of the product so that the signal components can be distinguished. By switching between many frequencies, it is expected that one frequency will provide a suitable sensitivity for any particular metal type, size, and orientation.
However, metal detection systems that operate at different frequencies typically have a lower sensitivity than systems that are tuned to a single frequency.
Hence, although signals of metal contaminants may be obtained with a desirable phase, the detection of these signals may still fail due to the low sensitivity of the metal detection system.
Also known in the art are metal detectors such as described in U.S. Pat. No. 6,724,191 B1, to Larsen, which discloses various circuits including an H-bridge switch network and a pulse width modulated switched capacitor resonator, for simultaneously resonating at several frequencies.
U.K. Patents GB 2423366 B and GB 2462212 B both refer to metal detectors that contain a drive circuit comprising four switches arranged as a full bridge circuit, wherein the coil system is connected across the output of the bridge. A programmable logic device controls the switches via a plurality of drive maps stored in the programmable logic device, with each drive map containing a switching sequence for a respective predetermined frequency of operation.
U.S. Pat. No. 5,859,533 to Gasnier describes an electromagnetic tomographic emitter for operating at variable frequencies to detect subsurface characteristics.
U.S. Pat. No. 5,304,927 discloses a method and apparatus for detection of metal in food products as packages of said food products are passed through the detector on a conveyor.
An exemplary embodiment of the present invention is therefore based on providing an improved method for operating a metal detection system that uses two or more transmitter frequencies as well as on providing a metal detection system adapted to operate according to this method.
Particularly, an exemplary embodiment provides a method that allows for detection of metal contaminants, particularly stainless steel contaminants, with high sensitivity, while signals caused by the product, the packaging, vibrations, or other potential disturbances are suppressed or eliminated.
More particularly, an exemplary embodiment provides an improved method for a metal detection system that allows the selection of numerous transmitter frequencies, preferably with small steps in the range from a few kHz to 1 MHz, or that generates square wave signals that comprise a large number of harmonics, for which signals with a desirable phase can be obtained for the metal contaminants.
An exemplary embodiment of the metal detection system comprises a balanced coil system with a transmitter coil and a first and a second receiver coil. The transmitter coil is connected to a transmitter unit, which generates transmitter signals having a transmitter frequency that is selected from a group of at least two transmitter frequencies. The first and the second receiver coil, that are coupled to each other, provide output signals to the signal input of at least one amplifier unit provided in a receiver unit. Due to the symmetrical arrangement of the receiver coils with respect to the transmitter coil and due to the inverse winding of the receiver coils, the signals induced in the receiver coils compensate one another in the absence of an external influence, such as a product, with or without contamination, or other disturbances such as vibrations. In this balanced state, the combined output signal of the receiver coils is zero.
According to an exemplary embodiment, a control unit provides a control signal, which depends on the transmitter frequency of the transmitter unit, to the control input of at least one controllable impedance unit. This controllable impedance unit is coupled to the signal input of the at least one amplifier unit, wherein the control signal is adapted to control the impedance value of the controllable impedance unit in such a way that the impedance value is increased or lowered according to the selected transmitter frequency.
By suitably varying the input impedance applied to the input of the amplifier in accordance with the selected transmitter frequency, the sensitivity of the metal detection system to contaminant metals is significantly improved. At the same time, a phase angle of signals originating from metalized film of packaging materials is kept close to 90° at any time.
In an exemplary embodiment, the receiver coils are coupled directly to the input of the amplifier unit via the controllable impedance unit. In another embodiment, the receiver coils are coupled to the primary windings of an input transformer, whose secondary windings are coupled via the controllable impedance unit to the input of the amplifier unit. The input transformer is used to isolate the amplifier unit galvanically from the receiver coils. Further, with a fixed or variable transmission ratio, a desirable voltage level of the input signal can be set.
In an exemplary embodiment, the receiver coils are connected with one tail to each other and with the other tail to the respective tails of two identical center-tapped primary windings of a balanced transformer. The balanced input transformer has two identical center-tapped secondary windings, whose opposite tails are connected to the input of the amplifier via the controllable impedance unit.
In a further embodiment, the controllable impedance unit comprises a transistor or a relay. The transistor can be employed as a switch to connect and disconnect a resistance to or from the input impedance amplifier circuit. In alternative embodiments a relay can be connected in parallel or in series with a resistor to vary the resistance value of the controllable impedance unit.
In an exemplary embodiment, preferably a low input impedance value is selected for low transmitter frequencies and a higher impedance value is selected for higher transmitter frequencies. With transmitter frequencies in the range 1 kHz to 300 kHz, input impedance in the range between 10 Ohm and 100 Ohm, most preferably close to 20 Ohm such as 22 Ohm, allows setting the phase of the signal originating from metalized film of the packaging material to about 90°. For transmitter frequencies above 300 kHz, low input impedance may unfavorably affect the sensitivity of metal contaminant signals. Therefore by increasing the input impedance value, the cut-off frequency of the low-pass filter formed by the head coil and the controllable impedance unit is also increased, and the gain at the required frequency is preserved. This can be realized with the selection of the input impedance above 100 Ohm, most preferably close to 300 Ohm, such as 330 Ohm.
In a second basic embodiment, the output signal of the receiver coils may be amplified and then filtered by means of a variable filter unit comprising at least one filter, whose centre frequency and filter bandwidth are adapted to the selected transmitter frequency, which may represent a carrier signal that has been modulated by the signals of the product and the metal contaminants.
Applying filters to the selected transmitter frequencies, i.e. carrier frequencies, leads to a further significant improvement of the sensitivity of the metal detection system.
Especially at frequencies below 300 kHz, where known metal detection system typically had a poor sensitivity and an unfavorable phase response of the signals derived from metal film packaging, a solution of an exemplary embodiment provides significant advantages. The appropriate band-pass filters favorably allow restoring the phase of the signals derived from metalized film of packaging material to 90 degrees. Especially with a transmitter signal frequency of 100 kHz, a filter with a cut-off frequency of 200 kHz, and for transmitter signal frequency of 200 kHz and 300 kHz, a filter with a cut-off frequency of 400 kHz, may achieve most favorable results. In an exemplary embodiment, that means that it brings the phase of the signals derived from metalized film of packaging material very close to 90 degrees.
With a band-pass filter, favorably a low-pass filter, the content of harmonic frequencies of the receiver signal may be removed, the signals within the bandwidth may be amplified, and the phase of signals derived from packaging metal film may be corrected close to 90 degrees. Consequently, this signal may be suppressed easily in an exemplary embodiment. An example of the applied band-pass filters may improve the phase performance of signals derived from packaging metal film at all frequencies, but an exemplary embodiment of a combination of the band-pass filter and a low impedance value of the controllable impedance unit may provide even better results at frequencies below 300 kHz.
In an exemplary embodiment, dedicated circuits may be used to amplify each signal with a different gain that depends on that signal phase relative to the phase of the transmitter signal phase. With this measure, an improvement of the sensitivity of the metal detection system (particularly to stainless steel materials) and a reduction of the sensitivity to disturbing vibrations may be achieved.
With the above methods that may advantageously be used independently or in combination, signals that originate from metalized film of packaging materials may be reduced while signals originating from metal contaminants may be detected with higher sensitivity.
In an exemplary embodiment, the measures allow the selected transmitter frequency of the received signal to pass to the phase sensitive detectors, while signals resulting from harmonic distortion are suppressed.
For a more accurate phase correction, the input amplifier unit may comprise more than two selectable impedance values. The filter may be made with a Butterworth, Chebyshev, Bessel, Cauer filter, or other low-pass filters and may be of a first or higher order. Each filter may have a different cut-off frequency, and in an exemplary embodiment it is preferably applied by means of a switch, e.g., a multiplexer, that is controlled in accordance with the selected transmitter frequency, so that the applied filter may remove the harmonic content from the receiver signal. With a transmitter frequency above 300 kHz, the signal of metalized film packaging may be obtained with a phase close to 90 degrees, which means that it may easily be suppressed.
To correct the phase of the signals derived from metalized film packaging in an exemplary embodiment, a low-pass filter is applied in the signal path between the amplifier unit that receives the input signal from the receiver coils and the phase sensitive detector. In this example, the applied filter improves the phase of the signals derived from metalized film packaging at all transmitter frequencies and reduces harmonic frequencies at transmitter frequencies below 300 kHz.
Advantageously, the low-pass filters in an exemplary embodiment are fifth order Butterworth filters that have a maximally flat amplitude response. Selecting a fifth order filter may allow obtaining a clearer pass-over between pass-band and stop-band.
According to another embodiment of the invention, the input amplifier comprises a bipolar cascode amplifier circuit coupled with a differential amplifier. An example of a cascode amplifier is stable, and has furthermore a high and linear gain, independently of the present frequency. Preferably, the amplifier is a differential amplifier comprising two amplifier units that amplify the signals present at the two opposite tails of the secondary windings of the input transformer.
The control unit preferably comprises a processing unit with a computer program that is designed to select the settings of the controllable impedance unit and/or settings of the variable filter according to an exemplary embodiment. The settings may be selected from a table provided in the control unit, containing at least a set of transmitter frequencies and corresponding settings for the at least one variable impedance unit and/or corresponding settings for the variable filter unit.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.